Present day disk drives employ a number of servo control techniques for controlling transducer positioning. Larger form-factor disk drives employ a dedicated servo surface on one disk platter to enable the generation of signals for servo control of head position. In such larger disk drives, this is a practical technique for servo generation since those drives contain multiple platters, yielding many surfaces. The dedication of one surface to servo signals subjects the disk drive to a "servo overhead" which is tolerable. For instance, in an eight platter drive, the "servo overhead", using a dedicated servo surface is only 6.25%.
Smaller form-factor disk drives (e.g. 31/2" or smaller) generally contain one or two disks. In such a drive, if one surface of a disk is dedicated to servo information, the overhead can be as high as 50% which is unacceptable. To minimize the servo overhead in these smaller disk drives, servo information has been embedded into the data tracks and is distributed throughout the track. Since the servo and data fields are thereby contiguous, differentiation between them becomes critical to proper operation of the disk drive.
Before a disk drive can be operated to store data, the disks must be formatted. Such formatting involves the partitioning of each recording surface into a plurality of pie-shaped sectors, each sector including a single servo burst. Since each disk track has a different linear velocity from every other track on the disk, signals written at a constant frequency do not exhibit a constant data density from track to track. For instance, a servo burst written at a constant frequency in a sector of an inner track will occupy less linear distance on the inner track than a constant frequency servo burst written on a more radially distant outer track. Nevertheless, servo bursts are written using constant frequency signals across the tracks. In such a case, radially aligned servo bursts occupy different linear distances in their respective tracks, but radially aligned servo bursts occupy an identical time window in each track (the angular velocity of each track is identical). Thus, a timing signal generated to window a servo burst and various signal segments therein can be made the same for each track irrespective of the radial distance of the track from a disk's spindle axis.
If constant frequency recording is used for data, as it is for servo bursts, the density of data in outer tracks is much less than for data in inner tracks and the resultant storage efficiency of the disk drive suffers. Many hard-disk disk drives therefore employ a method of recording termed "constant density" recording. More specifically, data is written in each track (or in a zone of tracks) so that the signal density is nearly constant across all tracks and more specifically from recording zone to recording zone. As a result, the frequency of recording of such data in greater radius tracks is higher than the frequency of recording in the more radially-inner tracks. Notwithstanding that data in larger radius tracks is recorded at a lesser density than data in smaller radius tracks, servo bursts are still recorded at a constant frequency. This fact enables timing control generators (used in constant density recording disk drives) to generate a single set of timing windows for all servo signals, irrespective of which track is being accessed.
In prior art disk drives, servo synchronization and timing-window generation circuitry has generally been of the hard-wired variety. If, during development, or for subsequent engineering change purposes, the formats of the servo or data fields were changed, a new timing generator was constructed and substituted for the outdated version. Such redesign and substitution required a long lead time and was expensive.
Accordingly, it is an object of this invention to provide a disk drive with a servo timing generator that is readily alterable to accommodate changed track formats.
It is another object of this invention to provide a disk drive servo timing generator that is programmably alterable.
It is yet another object of this invention to provide a programmable servo timing generator that may be reconfigured either statically or on-the-fly.